The attainment of high thermal efficiency is nearly always an important consideration in the field of power generation for the reason that the fuel cost is generally responsible for about two thirds of the cost of the power produced. In addition to the cost incentive, enviromental considerations require that greater effort be directed towards the achievement of higher efficiencies in order to minimise the production of carbon dioxide and other undesirable emissions.
In general it is possible to achieve a higher thermal efficiency and fewer emissions in large generating units than in small ones. This is partly because of heat losses, friction and leakage flows which tend to be proportionally less significant in large units than in small ones. Also economies of scale make it possible to have more sophisticated equipment in large units. In small units, the cost of such equipment may be prohibitive.
In spite of these factors, there are circumstances where small generating units are needed and it is important that they should be as efficient and enviromentally benign as possible. This situation arises in the many parts of the world where no electricity grid is available. It may be that construction of a power station to supply electricity is beyond the financial capacity of the local population or it may be that the electricity demand is too small to justify its construction. The former situation arises in many less developed countries. The latter situation applies in many remote or thinly populated regions and on offshore islands.
Another application for small efficient engines arises in connection with combined heat and power (CHP). The use of heat and power together usually results in a higher overall energy efficiency than the use of mains power from the electricity grid. Since heat cannot be transported economically over any significant distance, CHP systems have to be sized for the local heat load. This usually implies generating units of modest size.
The invention described here can be applied either as a heat engine or in modified form as a heat pump. Heat pumps transfer heat from a low temperature heat source to a high temperature heat sink. For example, in cold weather a heat pump can extract heat from the atmospheric air and pump it to a higher temperature in order to heat a building. Alternatively, in hot weather, the heat pump can operate as an air conditioning unit to extract heat from the internal air of the building and reject it to the outside atmosphere, even though the outside temperature is higher than the inside temperature. The heat pump may also be used to cool air in order to condense the water vapour in it. The heat rejected from the heat pump may then be used to restore heat to the air. In this case the heat pump is used to de-humidify the air. As with CHP, heat pumps have to be sized in accordance with the local heat load. Consequently, most heat pump capacity will be required in the form of small rather than large units.
Most types of heat pump, air conditioning unit or refrigeration system require the use of an evaporating/condensing fluid which boils at an appropriate temperature such as one of the chloro-fluoro-carbons (CFC's). These substances are known to deplete the earth's ozone layer which protects human and animal life from harmful ultra-violet radiation. Although certain alternatives to CFC's are known, some of these also cause ozone depletion, but to a lesser degree. Other alternatives have disadvantages such as flammability, toxicity, high cost, poor thermodynamic properties or a tendency to increase global warming.
Engines and heat pumps based on the Stirling Cycle are well known. One form of Stirling engine includes a compression chamber and an expansion chamber connected together via a regenerative heat exchanger forming a gas space which contains a working gas. According to the ideal Stirling Cycle working gas in the compression chamber is compressed by a piston and undergoes isothermal compression, the heat of compression being rejected to a low temperature heat sink. After this process is complete the cold working gas is pushed through the regenerator where it is preheated before entering the expansion chamber. In the expansion chamber, the hot compressed working gas is allowed to expand by forcing the piston out of the expansion chamber. During expansion, heat is added to the working gas so that the gas expands isothermally. The hot expanded gas is then pushed back through the regenerator to which it gives up its heat before being admitted to the compression chamber to begin the next cycle.
U.S. Pat. No. 4,148,195 describes a heat actuated heat pump which requires a high temperature heat source such as the combustion of fuel and another heat source at low temperature such as atmospheric air. The heat output is at an intermediate temperature. The purpose of the heat pump is to convert a certain amount of heat energy at high temperature to a larger amount of heat energy at the intermediate temperature. This is done by extracting heat energy from the low temperature heat source. The heat actuated pump described in U.S. Pat. No. 4,148,195 is a closed-cycle system without valves which approximates to the Stirling cycle. Liquid pistons contained in a series of four interconnected U-tubes and which are connected in a closed circuit displace the working gas between adjacent expansion and compression chambers formed in the arms of the U-tubes. The liquid pistons transmit power around the closed circuit directly from the expanding gas in the expansion chamber to the compressing gas in the adjacent compression chamber, an expansion chamber and a compression chamber being formed in opposed arms of the same U-tube. The four U-tubes are connected via the gas space with regenerators. Two of the four regenerators and the associated gas volumes work in a temperature range between the high temperature and the intermediate temperature. The other two regenerators and associated gas volumes work in a temperature range between the low temperature and the intermediate temperature. The cycle is operated in such a way that power is transmitted via the medium of the liquid pistons from the gas volumes working over the high temperature range to the gas volumes working over the low temperature range.
21st Inter-society Energy Conversion Engineering Conference Volume 1 (1986) pages 377 to 382 describes a Stirling heat actuated heat pump similar to that described in U.S. Pat. No. 4,148,195, in which the working gas is heated or cooled by taking liquid from a liquid piston, heating or cooling the liquid externally and reinjecting it into the expansion or compression cylinder as an aerosol.
One drawback of these known heat pumps is that the maximum working temperature of the high temperature heat source is very low in comparison to what can be achieved in modern advanced power generating technologies, such as the combined cycle gas turbine. For example the temperature of heat addition to the heat pump is likely to be limited to 400.degree. C., whereas the turbine inlet temperature of a modern power generating gas turbine is anything up to 1300.degree. C. Consequently the efficiency of conversion of the high temperature heat to internal work within the heat actuated heat pump is also low, as would be expected from considerations of Carnot's theorem. As a result the overall coefficient of performance is very low.
Another disadvantage of the heat actuated heat pump described in U.S. Pat. No. 4,148,195 lies in the fact that the liquid pistons have to be very long in order to achieve a low natural frequency of oscillation. The frequency of oscillation must be low because sufficient time must be allowed for heat transfer between the droplet spray and the gas. The required length of liquid piston is particularly difficult to achieve in a small device operating at high pressure. Also friction losses arising from long liquid pistons are likely to become unnacceptably high in a small device. Furthermore a high value for the ratio of length to stroke is required to avoid the so-called shuttle loss which arises from the transfer of heat from one end of each liquid piston to the other end. The shuttle loss occurs because the two ends of each liquid piston are at different temperatures and there is consequently some mixing of the liquid and transport of heat.
U.S. Pat. No. 3,608,311 describes an engine whose operation is based on the Carnot Cycle, in which gas is successively compressed and expanded in a single cylinder by a liquid displacer. Hot and cold liquid from the liquid displacer is alternately injected into the cylinder to heat the gas during part of the expansion process, and to cool the gas during part of the compression process.
One drawback of this known heat engine is that the power output per cycle is relatively low because it requires an extremely high compression ratio to raise the temperature of the working gas to a reasonable value during adiabatic compression, and such a compression ratio is not possible in practice. A further drawback of this engine is that the working gas is continually cycled between high and low temperatures while remaining in the same cylinder throughout the process. Therefore the walls of the cylinder also cycle from low to high temperatures and back again which implies large entropy changes and a reduction in thermodynamic efficiency.